Lymphocytes in culture are stimulated to undergo mitosis by several proteins including phytohemagglutinins and antibodies. We propose to (a) study some surface properties of lymphocytes before and during stimulation, (b) identify subpopulations of lymphocytes which are stimulated specifically by various lectins and antibodies, (c) isolate and identify surface receptor materials, (d) examine the role of other cell types and soluble lymphokines in stimulation, and (e) do a complete structural analysis on LcH-A, a homogeneous lectin from a common lentil. Using affinity chromatography and ion exchange chromatography, several homogeneous lectins with different specificities will be prepared and tagged with either a radiolabel or a fluorescent label. Quantitative binding of radiolabelled lectins to lymphocytes from inbred guinea pigs will be studied at various times in lymphocyte stimulation. The number of receptor sites and association constant of binding will be estimated from the measured number of labelled lectin molecules bound per lymphocyte with various concentrations of labelled lectin. Purification of receptor materials will be attempted by a (a) isolating pure lymphocytes, (b) lysing the lymphocytes and isolating surface membranes, (c) solubilizing surface receptors by disruption of lymphocyte surface membranes, and (d) passing the solubilized material over lectin-affinity columns and eluting the receptor materials with lectin-binding sugars or denaturing agents. Identification of subpopulations of lymphocytes will be done by (a) preparing antisera specific for certain subpopulations (such as T cells and/or B cells); (b) specifically lysing a particular subclass with specific antisera and complement, and (c) examining the surviving cells for stimulation by various materials. Soluble recruitment factors or lymphokines will be examined by stimulating cells with insolubilized stimulating lectins and examining the culture fluid for such factors. If they can be demonstrated to be present, attempt will be made to identify and isolate them by immunochemical techniques.